Temperature control for crystal pulling



Oct. 6, 1959 .1. LEVINSON 2,903,004

TEMPERATURE CONTROL FOR CRYSTAL PULLING Filed May 10, 1957 ELECTRONBALANCE D'FFERENT AMPLIFIER (ATOR fizuzcmomc DIFFERENT- AMPLIFIERBALANCE IATOR F I G 2 INVENTOR.

JOHN LEVlNSQN lwmm AT TORNEY United States Patent TEMPERATURE CONTROLFOR CRYSTAL PULLING John Levinson, Morristown, N.J., assignor to theUnited States of America as represented by the Secretary of the ArmyApplication May 10,1957, Serial No. 658,474 9 Claims. (Cl. 23--301) Thepresent invention pertains to a method and means for controlling therate of growth of a crystal in a crystal pulling furnace.

The growth of a crystal in a melt of crystal forming material is afunction of the temperature of the melt. The particles, such as atoms ormolecules, in the melt have a higher energy state than the sameparticles have in the solid state. They have a higher mobility and aremoving about in random direction and with momentum such that the weakerbinding forces operating between the particles cannot come into play andretain them in any definite order or pattern as present in the crystallattice. The temperature of the melt is a measure of the energy state ofthe particles. As the temperature of the melt is decreased oras thethermal energy of the particles becomes reduced, the binding forcesbetween the particles begin to dominate and the particles as they moveinto the pattern of the crystal lattice are retained and the solid stateis attained. The pattern of the crystal is characteristic of thematerial. Within limits, as the temperature is increased the number ofparticles at any given energy level decreases. Operating within theselimits more and more particles will be found in a given energy level asthe temperature is reduced. If the energy level be that which isfavorable to the attainment of the solid state, and the temperature isreduced, there will be an increase in the rate of growth of the crystal.On the other hand, the temperature is increased there will be a decreasein the rate of growth of the crystal. By controlling the temperature ofthe melt at the point where the process of crystallization is to takeplace, the rate of growth of the crystal can be controlled.

While temperature is an important factor in the rate of growth of acrystal, it is by no means the only factor. Impurities within the meltoperate to change the binding forces as the percentage changes withstratification of the melt. This and other possible factors thatinfluence the rate of growth make temperature unsatisfactory as thevariable to determine the rate of growth of the crystal.

Heretofore the art has used temperature as the variable factor and hassought to make it constant. A means inserted in the melt responds to thetemperature of the melt and produces a control effect which variestherewith. A control device inserted in a heat supply means exerts acontrol over the supply of heat for regulating the temperature of themelt. Between the temperature responsive means and the temperaturecontrol means a further means relates the response of the temperatureresponsive means and the needs of the control means operating tocounteract the change in temperature. These devices and methods forcontrolling the growth of the crystal do not overcome the variations ingrowth due to factors other than temperature. The devices and themethods are attended with further diificulties due to variations intemperature gradient between the point where crystallization is to becontrolled and the point where the temperature is taken.

The present invention seeks to overcome these difliculties in a mannerthat is productive of a more exact control over the growth of thecrystal. This makes possible the growing of crystals which are moreuniform in cross section and in texture, as the rate of removal of thecrystal when kept constant and the rate of growth of the crystal whenkept constant determine the cross section of the crystal.

In the present invention the temperature of the melt is utilized toinfluence and control the rate of growth of the crystal. It is varied asit becomes necessary to counteract the changes in the rate of growth ofthe crystal. The rate of growth of the crystal is reflected in the rateof change of the weight of the crystal on the one hand and in certaininstances in the rate of change in the weight of the melt of crystalforming material in the crucible. As the crystal increases in size itsweight increases and the weight of the melt decreases. The rate ofchange of weight is converted into a rate of change of voltage that isdiiferentiated to produce a voltage proportional to the rate of change.The voltage establishes a current having a value bearing a constantrelation to the voltage and producing a force that is proportional tothe rate of change of the weight of the crystal. The force is comparedwith and balanced against a constant force. As the variable force variesrelative to the constant force a control over the supply of heat isefiected to counteract the change in the rate of growth, and also tocounteract the change in the variable force. The responsiveness of themelt to the supply of heat, the re-' sponsiveness of the means measuringthe weight and the responsiveness of the intermediate links determinethe percentage of regulation that may be secured. By using the change ofweight of the crystal or the melt, those other factors which influencethe rate of growth of the crystal are counteracted by a change oftemperature and a more uniform growth of the crystal is possible.

An object of the invention is to provide a new method of growingcrystals in a crystal pulling furnace.

Another object of the invention is to provide improved means forcontrolling the rate of growth of a crystal in a crystal pullingfurnace.

Other objects of the invention will become obvious from consideration ofthe following specification, claims, and the accompanying drawings inwhich:

Fig. 1 is a view, partly diagrammatic and partly sectional, of oneembodiment of the invention; and

Fig. 2 is another view, partly diagrammatic and partly sectional showinganother embodiment of the invention.

There are no doubt other embodiments of the invention than those whichare disclosed herein which will operate to carry out the invention, butthe two embodiments of the present application will serve sufficientlywell to disclose the nature of the invention.

The crystal pulling furnace 1 is diagrammatically illustrated in Fig. 1.It consists of a crucible 2 for containing a melt of crystal formingmaterial and a heating means 2A associated therewith by which heat mightbe supplied to the crucible and the material therein. A seed crystal 3is attached to a holder 4 which is held by a chuck 5. The chuck 5 isconnected to the lower end of a rod 6 suspended from the arm 12. The arm12 and an associated arm 10 are guided for movement in a directionparallel to the length of the rod 6 whereby the rod 6 may be raised andlowered relative to the melt.

For raising and lowering the rod 6, a motor 7 that may be operated at acontrolled speed is connected to drive a threaded shaft 8 extendingparallel to the length of the rod 6 and engaging a threaded collar 9carried by the arm 10. Rotation of the threaded rod 8 causes thethreaded collar 9 to travel along the length of thethreaded rod 8 andcauses a companion movement of the arms and 12 in the guide to move therod 6 longitudinally thereof.

The crucible 2 is supported upon a table 13 mounted on a post 14 made ofheat insulating material. The lower end of the post "14 rests upon andis fixed to a metallic plate 15. The plate 15 rests upon a rubber ring16 that restsupon a metallic .plate 17. The ring 16 may be any yieldabledielectrical material which is able to support the weight of thecrucible, the melt and the table, and which will be compressedproportional to the weight. L1. The plates 15 and 17 form a capacitor18, the capacitance of which is Variable with the spacing of the plates,decreasing with the spacing and vice versa. The plates and the rubberring also form a weighing means producing a. change in capacitance withthe change in weight. As the crystal forms on the seed crystal theweight of the melt is decreased, the weight compressing the rubber ring16 becomes less and the rubber ring returns to an equal extent towardsits undeformed condition causing the separation of the plates of thecapacitor 18 and a correspending reduction ofthe capacitance thereof.

. The capacitor 18' is connected to the electronic balance 19'whereinthe change of capacitance is translated into a change in voltage. 'Theelectronic balance 19 is in turn connected to a differentiator 20 andthrough it to the coil 23 of the relay 22. The change in capacitance ofthe capacitor '18 causesa change of voltage to be produced at the outputof the electronic balance, the rate of change of the voltage being thesame as the rate of change of the capacitance and the rate of change ofthe weight of the melt. In the differentiator, the rate of change of theinput voltage is difierentiated and a voltage is producedthat causes acurrent in the coil 23 to be proportional to the rate of change of thevoltage fed to'the difierentiator.

The relay 22 also comprises a core and an armature 24 pivoted adjacentthe end of the core. The armature 24 operates as the movable contactcooperating with a fixed contact 26 to form a contactor. The contactoris in the circuit .27, 28 through which energy is supplied to theheating means 2A. V

- The armature 24 is biased by spring in opposition to the force exertedby the magnetic means comprising the coil 23and the core. The spring isconnected at one end to the armature 24 and at the other end to athreaded rod. The threaded rod passes through an aperture in a fixed armand has a knurled nut threaded thereon. The

threaded rod, fixed arm and knurled nut constitute a means for adjustingthe tension of the spring 25 by which standard of comparison can beselected and established. By adjusting the tension of the spring 25 aforce is established in opposition to the force exerted by the magneticmeans. The force exerted by' the magnetic means varies with the rate ofgrowth of the crystal. If the force exerted by the magnetic meansexceeds that exerted by the spring, and the force of the springrepresents the optimum rate of growth of the crystal, then it is obviousthat the rate of growth of the crystal exceeds the optimum rate ofgrowth, the contactor will be closed to supply more heat to the meltwhich has the effect of decreasing the rate of growth of the crystal.Vice versa, if the force exerted by the magnetic means is less than theforce exerted by the spring, it is apparent that the rate of growth ofthe crystal is less than the optimum rate of growth. The spring forcethen prevails and actuates the contactor to open position to discontinuethe supply of heat to the melt. The melt will cool by reason of normalradiation of heat and the rate of growth of the crystal will beincreased.

Another embodiment of the invention is disclosed in Fig. 2. Certainchanges of the structure shown in l, are required to care for theinstallation wherein the melt is prepared elsewhere than in the crucibleand is fed continuously to the crucible through a conduit. stance, theweight of the melt is unrelated to the growth In this inof the crystal.It therefore becomes necessary to obtain a continuous measurement of theweight of the crystal.

*In Fig. 2 the crucible 2 is supported upon a hot plate 2A whichfunctions merely to control the temperature of the melt during thecrystallization process so as to control the rate of growth of thecrystal. The heating means 2A is supplied through the circuit 27 and 23from a suitable source of electrical energy. In the circuit is thecontactor 24, 26 operable to control the supply of energy supplied tothe heating means.

I The crucible, as shown, is fed with a'continuous supply of meltmaterial through a conduit having a valve therein for controlling therate of supply. A seed crystal 3 is connected to a holder 4 carried by achuck 5 mounted on the lower end of the rod 6. In this embodiment therod 6 is constructed with a yieldable means that permits its length toincrease proportional with the increase in the weight of the crystalsupported thereby. This means comprises a. rod 6 having a cylindricalbore for housing a piston 6B,. A rod 6C is connected to the piston 6Band extends from the'cylindrical bore and is connectedto the arm 10.Surrounding the rod and interposed between" the piston and an end Wallof the cylindrical bore is a spring 16 which functions in the samecapacity as the rubber ring 16 in the embodiment shown in Fig. l.

Surrounding the rod 6 and affixed against longitudinal movement relativethereto is an arm 17A. Surrounding the rod 6C and aflixed againstlongitudinal movement relative thereto is an arm 15A. The arms 15A and17A preferably aremade of dielectric material and serve as ameans forsupporting the metallic plates 15 and 17. .The arms 15A and 17A areconnected respectively to metallic plates 15 and 17 by metallicrods. Theplates 15 and 17operate as a capacitor 18 in the same sense as theplates in Fig. 1. As the weight of the crystal increases, the plates 15and 17 separate because the spring 16 is compressed by the increase inweight. The separation of the plates 15 and 17 produces a decrease incapacitance. f I

The plates 15 and 17 are connected through the rods and conductors tothe electronic balance 19 wherein the change in capacitance istranslatedinto a change of voltage. p The electronic balance 19 is connected tothe differentiator 20, wherein the changing voltage is converted into avoltage that is proportional to the rate of change of the voltage'at theoutput of the electronic balance. The difterentiator 20 is connected tothe coil 23 or the relay 22 wherein a current produces a force that isproportional to the rate of change of the weight of the crystal. Therelay functions as in the previous embodiment, to control thesupply ofheat to the'melt and thus controls the temperature and the rate ofgrowth of the crystal.

"The. embodiments operate in' the manner already dis-- closed. A changein the rate of growth of the crystal is converted to a change in therate of change of a voltage.

' create a force that bears a .fixed relation to the voltage. In therelay the variable force is balanced against a'constant selected forceproduced by the spring. When the variable force varies and exceeds theforce exertedby the spring, the heating circuit is closed. When thevariable force. varies and becomes less than the force of the springthen the circuit is opened. While therelay is herein utilized as arn'eans for comparing a variable withafcon'stant as well as a meansforcontrol, it'is conceivable that other means maybe provided forestablishing a standard and other means may be provided for controllingthe supply of heat in responseto variations from the selected standard.7 g V g 7 Having described my invention and the best mode of operatingand nsing the samepwhat I consider to. be my invention is set forth inthe following claims: I a

'1. Means'for controlling the rate of growth of a crystal in a crystalpulling furnace comprising a crystal pulling furnace, heating means forsaid furnace, means for pulling a crystal from said furnace including aseed crystal thereon, means for yieldably supporting said furnacecomprising a heat insulating table, a post connected thereto, a pair ofplates separated by a yieldable means, one plate being supported by saidyieldable means on the other plate and supporting said post, said platesforming a variable capacitor, an electrical circuit connected to saidcapacitor for producing a variable voltage which varies as the variablecapacitor varies, a diiferentiator means connected to said electricalcircuit, an amplifier connected to said ditferentiator and a relayconnected to said amplifier said relay connected to the heater circuitfor controlling the heating current to saidfurnace.

2. Means for controlling the growth of a crystal comprising means forgrowing a crystal and means for controlling the rate of growth of thecrystal comprising a heating means, means for measuring the weight ofthe crystal as it is grown and means responsive to said last named meansfor controlling the supply of heat to the heating means for regulatingthe rate of growth of the crystal.

3. Means for controlling the rate of growth of a crystal comprisingmeans for growing a crystal, heating means for influencing the rate ofgrowth of the crystal, means for producing a voltage variable with thegrowth of the crystal, means for differentiating the voltage toestablish a voltage having a value that is proportional to the rate ofgrowth and means responsive to said voltage for controlling the amountof heat supplied to said heating means whereby the rate of growth of thecrystal may be maintained substantially constant.

4. Control means for controlling a crystal pulling furnace comprising acrucible for containing a body of melted material from which the crystalis to be formed, a heating means for said crucible for maintaining thebody of material at a temperature suitable for the formation ofcrystals, a seed crystal for initiating crystallization within said bodyof material, means for immersing said seed crystal within said body ofmelted material and for withdrawing it at a uniform rate of speed, meansresponsive to the weight of the melted material within the crucible forproducing a voltage proportional to the weight; means responsive to saidvoltage to produce a current proportional to the rate of change of saidvoltage for producing a force proportional thereto, means for producinga constant force and means responsive to the deviation of the variableforce from the constant force for controlling the amount of heatsupplied to the heating means, whereby the rate of growth of the crystalis maintained substantially constant.

5. Control means for controlling a crystal pulling furnace comprising acrucible for containing a body of material from which a crystal is to beformed, a heating means for said crucible for maintaining the body ofmaterial at a temperature at which crystallization will take place, aseed crystal, means for immersing the seed crystal and for withdrawingthe seed crystal from the melted material at a uniform rate, meansresponsive to the weight of the crystal being formed on the seed crystalfor producing a voltage which varies as the weight varies, meansconnected to said last named means for producing a current and a forceproportional to the rate at which said voltage varies, means forestablishing a constant force and means responsive to the constant forceand to the variable force and operable when said variable force deviatestherefrom, for controlling the supply of heat to said heating means,whereby the rate of growth of the crystal is maintained substantiallyconstant.

6. A method of growing crystals comprising the steps of providing a bodyof melted material from which a crystal is to be grown, heating saidmelted material to a temperature at which a crystal begins to form,inserting a seed crystal into said melted material to initiate theformation of a crystal thereon from the melted material, withdrawing theseed crystal and the crystal material forming thereon at a constant ratefrom the melted material, weighing the melted material to determine therate of growth of said crystal on said seed crystal, creating a voltageproportional to the rate of decrease of the weight of the meltedmaterial and controlling the supply of heat to said melted material inproportion to said voltage, whereby the rate of growth of the crystal isirnaintained constant commensurate with the rate of withdrawal of saidcrystal from the melted material.

7. A method of growing crystals comprising the steps of providing a bodyof melted material from which the crystal is to be formed; heating saidmaterial to a temperature at which a crystal will form; inserting a seedcrystal into said body of material to initiate the formation of acrystal thereon; withdrawing the seed crystal from the body of meltedmaterial commensurate with the rate of growth desired, measuring theweight of the crystal as it is grown; establishing a voltageproportional to the rate of growth of said crystal and controlling therate of heat supply to said body of melted material in proportion to thevoltage, whereby the rate of growth of said crystal is maintainedconstant.

8. An apparatus for growing crystals comprising means for heating amelted material to a temperature at which a crystal begins to form, aseed crystal to be inserted into the melted material, means forwithdrawing the seed crystal from the melted material at a constantrate, means for weighing the melted material, means responsive to theweighing means for determining the rate of growth of the crystal formedon the seed crystal and means responsive to the last mentioned means forcontrolling the heating means.

9. An apparatus for growing crystals comprising means for heating a bodyof melted materials, a seed crystal to be inserted into the meltedmaterial, means for withdrawing the seed crystal from the body of meltedmaterial at a constant rate, means for weighing the crystal formed onthe seed crystal, means responsive to the said weighing means forproducing a voltage proportional to the rate of growth of the crystaland means responsive to the said voltage to control the heat of the saidheating means whereby the rate of growth of said crystal is maintainedconstant.

References Cited in the file of this patent UNITED STATES PATENTS2,316,915 Truman Apr. 20, 1943 2,377,869 Elliott June 12, 1945 2,505,565Michel Apr. 25, 1950 2,525,587 Chan Oct. 10, 1950 2,739,297 AtanasoifMar. 20, 1956 OTHER REFERENCES Bell Tel. Lab., Transistor Technology,parts I and II, September 1952; pages 147 to 154, 174 to 180.

9. AN APPARATUS FOR GROWING CRYSTALS COMPRISING MEANS FOR HEATING A BODYOF MELTED MATERIALS, A SEED CRYSTAL TO BE INSERTED INTO THE MELTEDMATERIAL, MEANS WITHDRAWING THE SEED CRYSTAL FROM THE BODY OF MELTEDMATERIAL AT A CONSTANT RATE, MEANS FOR WEIGHING THE CRYSTAL FORMED ONTHE SEED CRYSTAL, MEANS RESPONSIVE TO THE SAID WEIGHING MEANS FORPRODUCING A VOLTAGE PROP TIONAL TO THE RATE OF GROWTH OF THE CRYSTAL ANDMEANS RESPONSIVE TO THE SAID VOLTAGE TO CONTROL THE HEAT OF THE SAIDHEATING MEANS WHEREBY TH ERATE OF GROWTH OF SAID CRYSTAL IS MAINTAINEDCONSTANT.